The present invention relates to adhesives, more specifically to pressure sensitive adhesives. In particular, the invention relates to pressure sensitive adhesives having favorable shear properties at elevated temperatures.
Pressure sensitive adhesive (PSA) compositions are used in a wide variety of applications, including many assembly and manufacturing applications. Numerous applications require PSAs to support a load at elevated temperatures, typically in the range of greater than 70xc2x0 C., for which high cohesive strength PSAs are required. A standard method of increasing cohesive strength at elevated temperatures is to chemically crosslink the PSA using irradiation processes, such as thermal radiation, ultraviolet (UV) radiation, gamma radiation, and electron beam (EB) radiation, etc. Although these processes improve cohesive strength, they often negatively impact other properties, including peel strength of the PSA.
A need exists for an improved PSA with high cohesive strength that does not require chemical crosslinking. The present invention is directed to PSA compositions and to PSA articles containing the adhesive composition. The adhesive compositions of the invention generally demonstrate desirable cohesive strength at elevated temperatures. This cohesive strength can be at least as high as that obtained with chemical crosslinking. Thus, the adhesive composition provides many of the advantages of crosslinking without various disadvantages, such as excessive degradation of the adhesive and loss of adhesion.
The PSA compositions generally contain an organic copolymer having pendant styrenic polymeric moieties. This copolymer is mixed with a polyarylene oxide polymer to provide an adhesive having favorable cohesive strength at elevated temperatures.
In general, the copolymer contains at least an acrylic acid ester of a non-tertiary alcohol and pendant styrenic polymeric moieties. The pendant styrenic moieties are believed to endow the acrylic polymeric backbone with elevated shear strength values compared to adhesives that do not contain the styrenic moieties. The copolymer is normally produced by adding or grafting the reinforcing polymeric moieties to the soft acrylic backbone to obtain the needed shear strength. The reinforcing polymeric moieties may be grafted by, for example, polymerizing a monomer with reactive sites located on the backbone, attaching preformed polymeric moieties to sites on the backbone, or by co-polymerizing the acrylic monomer with preformed polymeric monomer.
Copolymers suitable for use with the invention include those based on acrylic acid esters of non-tertiary alcohols. The acrylic acid esters are generally formed from an alcohol having from 1 to 14 carbon atoms, with the average number of carbon atoms being about 4 to 12 in certain implementations of the invention. However, in some implementations more than 12 carbon atoms may be present on average. As used herein, acrylic acid esters include, but are not limited to, acrylic and methacrylic acid esters. In one implementation the acrylic acid esters are isooctyl acrylate or 2-ethylhexyl acrylate. The acrylic acid esters can also optionally be copolymerized with a polar monomer to form a polymeric backbone. Suitable polar monomers include, without limitation, acrylic acid, methacrylic acid, itaconic acid, acrylamide, methacrylamide, acrylonitrile, methacrylonitrile, and combinations thereof.
The pendant styrenic polymeric moieties provide improvements in the shear strength of the PSA, and typically comprise polystyrene although other styrenic moieties may be used. For example, the pendant (graft) styrenic polymeric moieties may comprise (meth)acrylate-terminated polystyrene. In certain embodiments the pendant styrenic polymeric moieties have a molecular weight in the range of 2,000 to 30,000. Also, in some implementations the pendant styrenic polymeric moieties comprise from 1 to 30 weight percent of the total monomers of said copolymer, although amounts outside of this range are also possible.
The polyarylene oxide polymer provides improvements in the high-temperature performance of the adhesive composition. Generally the polyarylene oxide polymer comprises polyphenylene ether. For example, the polyarylene oxide polymer can include poly(2,6-dimethyl-1,4-phenylene ether). Typically the polyarylene oxide polymer has a glass transition temperature (Tg) of at least 100xc2x0 C., more typically of at least 120xc2x0 C., and even more typically of at least 140xc2x0 C. The polyarylene oxide polymer used in the PSA typically has a Tg that is at least about 20xc2x0 C. higher than that of the polymeric styrenic moieties of the acrylic copolymer. Surprisingly, it was found that in some embodiments, both peel and shear performance increased with increasing amounts of polyarylene oxide.
The polyarylene oxide generally has high thermodynamic selectivity for styrenic portions of the graft copolymer, resulting in an adhesive that demonstrates many of the desirable properties of a chemically crosslinked adhesive but with more versatility. In this manner, the ingredients of the adhesive are essentially xe2x80x9cphysicallyxe2x80x9d or xe2x80x9cstructurallyxe2x80x9d crosslinked by the aggregation of the styrenic moieties, without being covalently crosslinked. This physical or structural crosslinking is believed to be created by formation of a network of micro-phase separated domains formed by the hard styrenic blocks being swollen by the polyarylene oxide. The microphase-separated domains may have lamellar, spherical, cylindrical, micellar, co-continuous, or other morphologies.
The polymeric mixture comprising polyarylene oxide is generally suitable for use as an adhesive composition at elevated temperatures even without being chemically crosslinked. Thus, in most implementations, the polymeric mixture does not contain a chemical crosslinker and/or is not subjected to processes causing chemical crosslinking of the adhesive. However, in some implementations, the shear performance of the adhesive compositions of the present invention can be augmented by subjecting them to irradiation (e.g., actinic radiation, such as ultra-violet and thermal, and electron beam) or adding a chemical crosslinker, either of which can cause chemical crosslinking (i.e., covalent bonds).
In some preferred embodiments, the composition is not substantially chemically crosslinked. This can be shown, for example, by the gel content of the adhesive composition. In most implementations the adhesive composition has a gel content of about zero. The gel content is preferably less than 25 percent of the crosslinkable material, more preferably less than 10 percent, and most preferably less than 2 percent.
In other aspects, the invention is directed to a PSA composition comprising a graft copolymer having at least first and second monomers, wherein the first monomer is a monomeric acrylic or methacrylic acid ester of a non-tertiary alcohol, said alcohol having from 1 to 14 carbon atoms, with the average number of carbon atoms being about 4-12. The second monomer has the general formula Xxe2x80x94(Y)nxe2x80x94Z, wherein: X is a vinyl group copolymerizable with said first monomer; Y is a divalent linking group; where n can be zero or 1; Z is a monovalent styrenic polymeric moiety having a molecular weight in the range of about 2,000 to 30,000 and being essentially unreactive under copolymerization conditions. Optionally, a third monomer, a polar monomer such as acrylic acid, is included in the copolymer. The composition further includes at least some polyarylene oxide polymer. Various other materials, including polymers and monomers, may be incorporated into the composition.
In certain embodiments the PSA is formed into a foam that is characterized by a density lower than the density of the bulk PSA composition itself. Density reduction can be achieved in a number of ways, including, for example, through creation of gas-filled voids in the matrix (e.g., by means of a blowing agent) or inclusion of polymeric microspheres (e.g., expandable microspheres) or non-polymeric microspheres (e.g., glass microspheres). Embodiments that contain the expanded polymeric microspheres create a foam article with numerous voids. The interiors of the voids are preferably substantially non-adhesive, and are therefore compressible without collapsing. In this manner the foamed PSA articles can be subjected to routine pressure without excessive degradation of their foam properties.
The invention is also directed to a method of forming a PSA article, including a foamed PSA. At least one method embodiment includes providing a polymeric composition containing a copolymer as described herein and a polyarylene oxide; heating the polymeric composition to above a Tg without substantially degrading the polymeric components; mixing the polymeric composition; cooling the polymeric composition to a temperature below the activation temperature of polymeric microspheres, and adding expandable polymeric microspheres to the cooled polymeric composition. The method generally further comprises heating the polymeric composition above the activation temperature of the polymeric microspheres.
Other features and advantages of the invention will be apparent from the following detailed description of the invention and the claims. The above summary of principles of the disclosure is not intended to describe each illustrated embodiment or every implementation of the present disclosure.